Development workflow

You already have your own forked copy of the ODL repository by following Making your own copy (fork) of ODL. You have Set up your fork. You have configured Git according to Configure Git. Now you are ready for some real work.

Workflow summary

In what follows we’ll refer to the upstream ODL master branch, as “trunk”.

  • Don’t use your master branch for anything. Consider deleting it.

  • When you are starting a new set of changes, fetch any changes from trunk, and start a new feature branch from that.

  • Make a new branch for each separable set of changes — “one task, one branch” (see IPython Git workflow).

  • Name your branch for the purpose of the changes - e.g. issue-128__performance_tests or refactor_array_tests. Use the issue-<number>__ prefix for existing issues.

  • If you are fixing a bug or implement a new feature, consider creating an issue on the ODL issue tracker first.

  • Never merge trunk or any other branches into your feature branch while you are working.

  • If you do find yourself merging from trunk, Rebase on trunk instead.

  • Ask on the ODL mailing list if you get stuck.

  • Ask for code review!

This way of working helps to keep the project well organized, with readable history. This in turn makes it easier for project maintainers (that might be you) to see what you’ve done, and why you did it.

See Linux Git workflow and IPython Git workflow for some explanation.

Consider deleting your master branch

It may sound strange, but deleting your own master branch can help reduce confusion about which branch you are on. See deleting master on GitHub for details.

Update the mirror of trunk

First make sure that Linking your repository to the upstream repo is done.

From time to time you should fetch the upstream (trunk) changes from GitHub:

$ git fetch upstream

This will pull down any commits you don’t have, and set the remote branches to point to the right commit. For example, “trunk” is the branch referred to by (remote/branchname) upstream/master - and if there have been commits since you last checked, upstream/master will change after you do the fetch.

Make a new feature branch

When you are ready to make some changes to the code, you should start a new branch. Branches that are for a collection of related edits are often called “feature branches”.

Making an new branch for each set of related changes will make it easier for someone reviewing your branch to see what you are doing.

Choose an informative name for the branch to remind yourself and the rest of us what the changes in the branch are for, for example add-ability-to-fly or issue-42__fix_all_bugs.

Is your feature branch mirroring an issue on the ODL issue tracker? Then prepend your branch name with the prefix issue-<number>__, where <number> is the ticket number of the issue you are going to work on. If there is no existing issue that corresponds to the code you’re about to write, consider creating a new one. In case you are fixing a bug or implementing a feature, it is best to get in contact with the maintainers as early as possible. Of course, if you are only playing around, you don’t need to create an issue and can name your branch however you like.

  # Update the mirror of trunk
$ git fetch upstream
  # Make new feature branch starting at current trunk
$ git branch my-new-feature upstream/master
$ git checkout my-new-feature

Generally, you will want to keep your feature branches on your public GitHub fork of ODL. To do this, you git push this new branch up to your GitHub repo. Generally (if you followed the instructions in these pages, and by default), Git will have a link to your GitHub repo, called origin. You push up to your own repo on GitHub with

$ git push origin my-new-feature

In git >= 1.7 you can ensure that the link is correctly set by using the --set-upstream option:

$ git push --set-upstream origin my-new-feature

From now on Git will know that my-new-feature is related to the my-new-feature branch in the GitHub repo.

The editing workflow


  # hack hack
$ git add my_new_file
$ git commit -m "BUG: fix all bugs"
$ git push

In more detail

  1. Make some changes.

  2. See which files have changed with git status (see git status). You’ll see a listing like this one:

    On branch my-new-feature
    Changed but not updated:
      (use "git add <file>..." to update what will be committed)
      (use "git checkout -- <file>..." to discard changes in working directory)
            modified:   README
    Untracked files:
      (use "git add <file>..." to include in what will be committed)
    no changes added to commit (use "git add" and/or "git commit -a")
  3. Check what the actual changes are with git diff (see git diff).

  4. Add any new files to version control git add new_file_name (see git add).

  5. To commit all modified files into the local copy of your repo, do git commit -am "A commit message". Note the -am options to commit. The m flag just signals that you’re going to type The commit message on the command line. The a flag — you can just take on faith — or see why the -a flag? — and the helpful use-case description in the tangled working copy problem. The git commit manual page might also be useful.

  6. To push the changes up to your forked repo on GitHub, perform a git push (see git push).

The commit message

Bear in mind that the commit message will be part of the history of the repository, shown by typing git log, so good messages will make the history readable and searchable. Don’t see the commit message as an annoyance, but rather as an important part of your contribution.

We appreciate if you follow the following style:

  1. Start your commit with an acronym, e.g., BUG, TST or STY to indicate what kind of modification you make.

  2. Write a one-line summary of your modification no longer than 50 characters. If you have a hard time summarizing you changes, maybe you need to split up the commit into parts.

    Use imperative style, i.e. write add super feature or fix horrific bug rather than added, fixed .... This saves two characters for something else.

    Don’t use markdown. You can refer to issues by writing #12. You can even have GitHub automatically close an issue by writing closes #12. This happens once your commit has made its way into master (usually after merging the pull request).

  3. (optional) Write an extended summary. Describe why these changes are necessary and what the new code does better than the old one.

Ask for your changes to be reviewed or merged

When you are ready to ask for someone to review your code and consider a merge:

  1. Go to the URL of your forked repo, say

  2. Use the “Switch branches/tags” dropdown menu near the top left of the page to select the branch with your changes:

  3. Click on the “New Pull Request” button:


    Enter a title for the set of changes, and some explanation of what you’ve done. Say if there is anything you’d like particular attention for - like a complicated change or some code you are not happy with.

    If you don’t think your request is ready to be merged, just say so in your pull request message. This is still a good way of getting some preliminary code review.

    See also:

Some other things you might want to do

Delete a branch on GitHub

$ git checkout master
  # delete branch locally
$ git branch -D my-unwanted-branch
  # delete the remote branch on GitHub
$ git push origin :my-unwanted-branch

Note the colon : before test-branch.

See also:

Several people sharing a single repository

If you want to work on some stuff with other people, where you are all committing into the same repository, or even the same branch, then just share it via GitHub.

First fork ODL into your account, as from Making your own copy (fork) of ODL.

Then, go to your forked repository GitHub page, say

Click on “Settings” -> “Collaborators” button, and invite other people the repo as a collaborator. Once they have accepted the invitation, they can do

$ git clone

Remember that links starting with git@ use the ssh protocol and are read-write; links starting with https:// are read-only.

Your collaborators can then commit directly into that repo with the usual

$ git commit -am "ENH: improve code a lot"
$ git push origin master  # pushes directly into your repo

See also:

Explore your repository

To see a graphical representation of the repository branches and commits, use a Git GUI like gitk shipped with Git or QGit included in KDE:

$ gitk --all

To see a linear list of commits for this branch, invoke

$ git log

You can also look at the Network graph visualizer for your GitHub repo.

Finally the Fancy log output fancylog alias will give you a reasonable text-based graph of the repository.

Rebase on trunk

Let’s say you thought of some work you’d like to do. You Update the mirror of trunk and Make a new feature branch called cool-feature. At this stage trunk is at some commit, let’s call it E. Now you make some new commits on your cool-feature branch, let’s call them A, B, C. Maybe your changes take a while, or you come back to them after a while. In the meantime, trunk has progressed from commit E to commit (say) G:

      A---B---C cool-feature
D---E---F---G trunk

Now you consider merging trunk into your feature branch, and you remember that this page sternly advises you not to do that, because the history will get messy. Most of the time you can just ask for a review, and not worry that trunk has got a little ahead. But sometimes, the changes in trunk might affect your changes, and you need to harmonize them. In this situation, you may prefer to do a rebase.

Rebase takes your changes (A, B, C) and replays them as if they had been made to the current state of trunk. In other words, in this case, it takes the changes represented by A, B, C and replays them on top of G. After the rebase, your history will look like this:

              A'--B'--C' cool-feature
D---E---F---G trunk

See rebase without tears for more detail.

To do a rebase on trunk:

  # Update the mirror of trunk
$ git fetch upstream

  # go to the feature branch
$ git checkout cool-feature

  # make a backup in case you mess up
$ git branch tmp cool-feature

  # rebase cool-feature onto trunk
git rebase --onto upstream/master upstream/master cool-feature

In this situation, where you are already on branch cool-feature, the last command can be written more succinctly as

$ git rebase upstream/master

When all looks good you can delete your backup branch:

$ git branch -D tmp

If it doesn’t look good you may need to have a look at Recovering from mess-ups.

If you have made changes to files that have also changed in trunk, this may generate merge conflicts that you need to resolve - see the git rebase manual page for some instructions at the end of the “Description” section. There is some related help on merging in the Git user manual - see resolving a merge.

Recovering from mess-ups

Sometimes, you mess up merges or rebases. Luckily, in Git it is relatively straightforward to recover from such mistakes.

If you mess up during a rebase:

$ git rebase --abort

If you notice you messed up after the rebase:

  # reset branch back to the saved point
$ git reset --hard tmp

If you forgot to make a backup branch:

  # look at the reflog of the branch
$ git reflog show cool-feature

8630830 cool-feature@{0}: commit: BUG: io: close file handles immediately
278dd2a cool-feature@{1}: rebase finished: refs/heads/my-feature-branch onto 11ee694744f2552d
26aa21a cool-feature@{2}: commit: BUG: lib: make seek_gzip_factory not leak gzip obj

  # reset the branch to where it was before the botched rebase
$ git reset --hard cool-feature@{2}

Rewriting commit history


Do this only for your own feature branches.

There’s an embarrassing typo in a commit you made? Or perhaps the you made several false starts you would like the posterity not to see.

This can be fixed via interactive rebasing.

Suppose that the commit history looks like this:

$ git log --oneline
eadc391 Fix some remaining bugs
a815645 Modify it so that it works
2dec1ac Fix a few bugs + disable
13d7934 First implementation
6ad92e5 * masked is now an instance of a new object, MaskedConstant
29001ed Add pre-nep for a copule of structured_array_extensions.

and 6ad92e5 is the last commit in the cool-feature branch. Suppose we want to make the following changes:

  • Rewrite the commit message for 13d7934 to something more sensible.

  • Combine the commits 2dec1ac, a815645, eadc391 into a single one.

We do as follows:

  # make a backup of the current state
$ git branch tmp HEAD
  # interactive rebase
$ git rebase -i 6ad92e5

This will open an editor with the following text in it:

pick 13d7934 First implementation
pick 2dec1ac Fix a few bugs + disable
pick a815645 Modify it so that it works
pick eadc391 Fix some remaining bugs

# Rebase 6ad92e5..eadc391 onto 6ad92e5
# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
# If you remove a line here THAT COMMIT WILL BE LOST.
# However, if you remove everything, the rebase will be aborted.

To achieve what we want, we will make the following changes to it:

r 13d7934 First implementation
pick 2dec1ac Fix a few bugs + disable
f a815645 Modify it so that it works
f eadc391 Fix some remaining bugs

This means that (i) we want to edit the commit message for 13d7934, and (ii) collapse the last three commits into one. Now we save and quit the editor.

Git will then immediately bring up an editor for editing the commit message. After revising it, we get the output:

[detached HEAD 721fc64] FOO: First implementation
 2 files changed, 199 insertions(+), 66 deletions(-)
[detached HEAD 0f22701] Fix a few bugs + disable
 1 files changed, 79 insertions(+), 61 deletions(-)
Successfully rebased and updated refs/heads/my-feature-branch.

and the history looks now like this:

0f22701 Fix a few bugs + disable
721fc64 ENH: Sophisticated feature
6ad92e5 * masked is now an instance of a new object, MaskedConstant

If it went wrong, recovery is again possible as explained above.